Information storage systems, particularly computer memory systems, typically store data magnetically or optically onto several types of media, such as rotating disks. Data stored on such disks, whether magnetic or optical, is contained within a series of tracks. Once formed on a disk, such tracks are generally concentric shaped and can number from approximately 77 to several thousand tracks, depending on the diameter of the disk utilized and whether the information is recorded magnetically or optically. The tracks on a disk can be viewed as roughly analogous to grooves on a phonograph record.
In magnetic recording and magneto-optical recording, information is stored on a subject media by orienting the magnetic field of the media at given points along given tracks. In order to access or read data stored on a disk, a so-called head or transducer is moved along a generally radial path across the surface of the disk as the disk is spinning. The generally radial movement will either follow a straight line path or an arcuate path, depending on whether a linear or rotary actuator is used for positioning the head.
Generally, actuators of the type described above affect movement of the transducer through a realization of the principle that if current is passed in a first direction through a conductor, which conductor is positioned in a magnetic field having a second direction, a force will be generated acting on the conductor in a third direction. This principle has been known as the right-hand rule. In such devices a sliding or rotating carrier is provided with a coil. A portion of the coil passes through a magnetic field. As current is applied in one direction or another through the coil, the carrier will move in a corresponding direction.
An illustration of this principle can be found in U.S. Pat. No. 4,607,913, issued to Jansen, wherein a linear actuator is described for moving a lens system along a radial path with respect to an optical disk. The patent discloses a box-like frame having independent upstanding walls defining a generally rectangular opening. Permanent magnets are shown to be attached to walls on opposite sides of the opening and a pair of guide rods are attached which pass between the magnets. An axially slidable carrier is shown mounted on the guide rods. A magnetic field created by this device included flux directed from the permanent magnet to the guide rod where upon it was conducted to the frame and back to the permanent magnet. A coil mounted on the carrier included a portion which passed through this magnetic field. By passing current through the coil, the carrier was said to move axially along the guide rods.
Another example of such an actuator was described in Yamamoto, T., "Development Of High Performance Head Positioner For An Optical Disk Storage System", S.P.I.E. Vol. 695, Optical Mass Data Storage II (1986) pages 153-159. Yamamoto describes a linear actuator for moving an optical lens system along a radial path with respect to an optical disk. The positioner was said to consist of a carriage mounted in an axially slidable fashion on a linear guide system which included two generally parallel guide rails. A single voice coil motor mounted to the carriage extends a distance on either side of the carriage and guide rail arrangement creating a large loop. Positioned on either side of the carrier and encompassing a portion of the voice coil were magnetic circuits. The magnetic circuits included structure similar to that shown in FIG. 1 wherein two flux conductors 10 and 12 are shaped to define a linear gap 14 therebetween. A permanent magnet 16 is attached to flux conductor 10 and extends partially across the width of gap 14 and extends substantially along the length of gap 14. Magnetic circuit structure similar to that shown in FIG. 1 was provided to both sides of the positioner described in Yamamoto. The voice coil passed through gap 14 in each structure. Consequently, when a current was passed through the voice coil, a force was applied to the coil which resulted in movement of the carriage along the linear guide system.
The problem with both Yamamoto and Jansen lies in the inefficiency of the magnetic circuit. Referring to FIG. 1, it will be understood that flux generated by magnet 16 will be directed across gap 14 to conductor 12 whereupon the flux will be conducted through conductor 10 and back to magnet 16. The magnetic field orientation of magnet 16 will determine the direction of flux across gap 14. In such a circuit, flux density is greatest at the contacting surfaces between conductor 10 and conductor 12. These surfaces by necessity create a small air gap, which will be dependent upon the preciseness by which the surface is machined. Consequently, the flux density in gap 14 and thus the efficiency of the actuator is affected by the care by which these components are made. Moreover, since magnet 16 is attached to conductor 10 during an assembly operation, large attractive forces will exist between conductor 10 and conductor 12 making assembly of the structure difficult. The combination of large attractive forces and precisely machined parts adds the possibility of worker injury during assembly.
However, a break in the structure of the flux conductor is necessary in order to economically construct the actuator. If the flux conductor were continuous throughout, i.e., conductors 10 and 12 were to be integrally formed, the coil could only be assembled so that a portion passed through gap 14 by winding the coil through the gap during coil assembly. Such a manufacturing operation is both cumbersome and uneconomical.
Consequently, a need exists in the art for magnetic circuit structure which both maximizes efficiency and provides for ease of assembly when used in an actuator device.